In many semiconductor integrated circuit fabrication processes, a contact is formed to electrically connect an active region formed in a semiconductor substrate with a metal interconnect line formed on a dielectric layer disposed between the interconnect line and the substrate. In forming the contact, a contact hole is typically formed in the dielectric layer to expose the active region, with a conductive plug providing the inter-layer conductive path from the active region to the interconnect line. A barrier layer is commonly formed between the active region and the conductive plug formed in the contact hole. The barrier layer helps reduce interdiffusion of metal from the conductive plug and silicon from the active region, which typically occurs through a silicide formed on the upper surface of the active region. Ideally, this barrier layer adheres to both the silicide and the metal, prevents interdiffusion, and covers the contact hole in a uniform conformal manner.
In practice, the stepped character of the contact hole prevents the uniform conformal formation of the barrier layer within the contact hole. One reason for this problem is that the barrier layer is typically formed by noncollimated sputter deposition. As is well known in the art of integrated circuit fabrication, the relatively high aspect ratio of a typical contact hole causes poor step coverage for noncollimated sputter deposition. Generally, as the aspect ratio increases, the step coverage worsens. In addition, the poor step coverage usually results in "overhang" that undesirably narrows the opening of the contact hole (described below in conjunction with FIG. 1). Conventional solutions to this problem include reducing the slope of the step or the thickness of barrier metal film. However, these solutions are not optimal because reducing the slope increases the area the contact occupies and reducing the thickness of the barrier layer also reduces the effectiveness of the barrier layer in preventing interdiffusion.
FIG. 1 shows a cross-sectional view of a stage in the conventional fabrication of a barrier layer in a contact hole. A dielectric layer 14 is formed on top of a substrate 10 having a conductive area 12. The conductive area 12 can be a source/drain region or gate electrode of a metal oxide semiconductor (MOS) transistor. The term MOS is used herein to also refer to silicon gate technologies. The dielectric layer 14 can be an oxide or a borophosphosilicate glass (BPSG) that is deposited on the substrate 10 in any suitable manner.
A contact hole 17 is then formed in the dielectric layer 14 using any suitable technique. For example, conventional patterning and etching processes can be performed to pattern the dielectric layer 14 and form the contact hole 17 aligned with the conductive area 12. As a result, a portion of the conductive area 12 is exposed. The contact hole 17 typically has a relatively high aspect ratio of about two or higher.
A barrier metal layer or film 16 is then formed on the dielectric layer 14 and in the contact hole 17. As stated above, the barrier metal layer 16 is typically formed by noncollimated sputter deposition. In this example, the barrier metal is a stack of titanium and titanium nitride (i.e., Ti/TiN). In other embodiments, other metal stack structures can be used such as, for example, W/AlSiCu/TiN. The Ti/TiN barrier layer has a thickness in the range about 300 .ANG.-500 .ANG. for the Ti and about 1300 .ANG.-1500 .ANG. for the TiN.
As shown in FIG. 1, the opening of contact hole 17 has an overhang profile. The overhang 15 undesirably narrows the opening of the contact hole 17, which in effect increases the aspect ratio of the contact hole 17. Then, tungsten 18 is deposited over the barrier layer and into the contact hole 17. Because of the narrow opening and the shape of the contact hole 17, the tungsten does not completely fill the contact hole 17 as shown in FIG. 1A. As a result, the tungsten plug is thin, which causes the tungsten plug to have a relatively high resistance. Of course, high resistance is generally undesirable in a contact.